This invention relates to electroluminescent (EL) lamps and, in particular, to an EL lamp having a heater element as one electrode As used herein, an EL xe2x80x9cpanelxe2x80x9d is a single substrate including one or more luminous areas, wherein each luminous area is an EL xe2x80x9clamp.xe2x80x9d One or more lamps in a panel can include a heater as an electrode.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder emits light in the presence of a strong electric field, using very little current.
A modern EL lamp is a thick film device, typically including a transparent substrate of polyester or polycarbonate material having a thickness of about 7.0 mils (0.178 mm.). A transparent, front electrode of indium tin oxide (ITO) or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 xc3x85 or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. Other methods for making an EL lamp can be used instead, e.g. roll coating.
The inks used for making an EL lamp include a binder, a solvent, and a filler, wherein the filler determines the nature of the printed layer. A typical solvent is dimethylacetimide (DMAC) or ethylbutylacetate (EB acetate). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, or epoxy. A phosphor layer is typically deposited from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically deposited from a slurry containing a solvent, a binder, and barium titanate (BaTiO3) particles. A rear (opaque) electrode is typically deposited from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon. Because the solvent and binder for each layer are chemically the same or similar, there is chemical compatibility and good adhesion between adjoining layers.
In many applications, e.g. automotive displays and portable devices such as wrist watches, radios, global positioning systems, and compact disk players, an EL lamp is used for backlighting a liquid crystal display (LCD); e.g., as disclosed in U.S. Pat. No. 4,580,877 (Washo) and U.S. Pat. No. 5,121,234 (Kucera). The combination is popular because of the uniformity of the back lighting and the resolution and contrast of the LCD. There is a problem in that the liquid crystal material becomes too viscous for proper operation at low temperatures, e.g., below 0xc2x0 C. It is known to provide a heaters for an LCD but a heater often increases the bulk of the display, which is undesirable. Also, heaters typically apply heat nonuniformly to the display.
It is known in the art to vary the resistivity of an ITO layer by changing thickness As disclosed in U.S. Pat. No. 4,880,475 (Lindmayer), relating to a thin film (glass substrate) electroluminescent device; an ultra-thin, high resistivity ITO layer is protected by another metal oxide layer, such as nickel oxide.
It is also known in the art to use an ITO layer as a heater in an LCD. As disclosed in U.S. Pat. No. 5,559,614 (Urbish et al.), the heater electrode is coplanar with but separate from the rear electrode in the LCD. The heater electrode is disclosed as a combination of ITO and a copper oxide layer. Connections to the heater are only described generically but at least two additional connectors are required for the heater. More than two additional connections are likely required for a patterned heater electrode.
It is also known in the art for an LCD and an EL lamp to share a common electrode, e.g. as disclosed in U.S. Pat. No. 4,500,173 (Leibowitz et al.), but it is not known that the common electrode be a heater.
In view of the foregoing, it is therefore an object of the invention to provide an EL lamp with a heater electrode.
Another object of the invention is to provide an EL lamp wherein the front electrode and the heater electrode are the same electrode.
A further object of the invention is to provide an EL lamp wherein the rear electrode and the heater electrode are the same electrode.
Another object of the invention is to provide an EL lamp with an integral heater requiring a single, additional connector.
A further object of the invention is to provide a liquid crystal display with an EL lamp for backlighting and for heating the liquid crystal display.
Another object of the invention is to provide a liquid crystal display backlit by an EL lamp having a transparent, heating, front electrode.
A further object of the invention is to provide a liquid crystal display with a heater covering the entire area of the display.
The foregoing objects are achieved in this invention wherein an electroluminescent lamp includes two electrodes and at least three connectors, a first connector coupled to a first electrode and a second connector and a third connector coupled to the second electrode. A first electric current flows through the lamp between the first connector and the second connector, causing the lamp to emit light. A second electric current flows through the second electrode between the second connector and the third connector, whereby the second electrode increases in temperature in response to the second electric current. When backlighting a liquid crystal display, the second electrode provides heat to warm the display for proper operation. Either electrode can be used as a heater.